(a) The Field of Invention
This invention relates to electric motors and more particularly to armature constructions having permanent magnets that provide magnetic fields for interaction with the magnetic flux of the motor field windings.
(b) The Prior Art
It has been common to employ permanent magnets in electric motors to establish fixed magnetic fields. One application is in small D.C. motors where they are used as field pieces that surround a wound armature to which a D.C. voltage is applied. These field pieces are arcuately shaped, and are circumferentially spaced from one another in closely conforming relation to the armature surface to form an air gap therebetween. Typically, they are mounted on the inside of a magnetic shell which acts as a return path for the magnetic flux of the magnets. The magnets will be alternately polarized with the inner pole faces of the magnets facing the armature being of alternate polarity, so that flux emanating from one magnet will pass through the armature and then return into the circumferentially adjacent magnets. Thus, a pair of magnets are required for each pair of magnetic poles.
Another application of permanent magnets in electric motors is for synchronous A.C. motors. Here, the permanent magnets are usually mounted in a rotating armature that is surrounded by the A.C. field windings of the motor stator. These motors find use in drive systems for diverse applications such as in film processing, the fiber industry, and the like. Also, by varying the input frequency to the field windings variable speed drive systems can be provided.
In prior A.C. synchronous motors the permanent magnets have been arranged in such fashion that it requires a pair of magnets to develop a pair of magnetic poles. In a two pole design, for instance, a pair of arcuate shaped magnets are embedded in the armature which are diametrically opposite one another. The outer face of one magnet presents a positive magnetic pole and the outer face of the other magnet presents a negative magnetic pole. The magnetic flux path has the two magnets in series relation, and the flux path extends diametrically across the armature to include the region of the motor shaft. In many applications the shaft becomes magnetized, or is of a special non-magnetic material to avoid magnetization.
In another A.C. synchronous motor design the permanent magnets in the rotor are circularly spaced from one another and their magnetic pole faces are arranged to face circularly within the rotor, instead of in a radial direction. Poles of like polarity of adjacent magnets face one another, so that the magnetic flux pattern will develop poles at the armature surface which are centered between magnets. In such a structure the magnetic flux of the permanent magnets also tends to develop return paths through the central core of the armature, as well as across the working air gap of the motor, and special design is required to block such unwanted flux paths in the central regions of the armature.